The focus of this project continues to be on certain aspects of the immune response to infection with Strongyloides stercoralis in humans as well as in the experimental host, the Patas monkey. A strong but not complete degree of protection to challenge was demonstrated in several previously infected Patas monkeys. The presence of intestinal mast cells appears to be a prominent feature of chronic strongyloides infection in the monkey. Increasing experience with the immediate skin test composed of somatic and metabolic larval antigens indicates that it is useful for diagnostic purposes in human infections. Reaction sizes for positive tests have been determined. Analysis of larval antigens using polyacrylamide gel electrophoresis and immuno-blotting has been initiated.